thumbnails: allow choosing between random object in folder or first object in folder

[tkimnguyen]

Currently in the PDF Group View if a contained item is a folder, the thumbnail for that folder is randomly chosen from the folder's contained image or file objects.

However this means that what gets shown for thumbnails is unpredictable.

Would be nice to allow choosing between the current behaviour and always taking the thumbnail from the first image or file in the contained folder (maybe recursively?)